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Spotlight on Optics

January 2013

Spotlight Summary by Taek Yong Hwang

The wettability of a solid surface is governed by the precise balance of surface tensions of all the interfaces (liquid/vapor, liquid/solid, and vapor/solid interfaces), as described by Young’s equation. This balance can be altered by chemical or physical modifications of the surface, and this change in the balance can be experimentally monitored through measurements of the contact angle. With advancements in laser processing technology, the wettability of the surface can also be controlled with laser irradiation through surface structuring on various materials including semiconductors, metals, and dielectrics. Compared with the control of surface wettability through chemical modifications, surface structuring with laser irradiation can be preferable because it leads to more stable surface properties with no dramatic degradation. It is also possible to selectively control the wettability of a surface as small as a tightly focused laser spot or as large as needed. Additionally, previous studies showed asymmetric features on the wettability of the surface, where a more hydrophilic property can be achieved on dielectrics and semiconductors in the direction of laser beam scanning, and this can be potentially useful to conventional fluidic devices and optofluidic devices.

Recently, the authors demonstrated the self-assembly of optical microwires using convective flow within a drop of water-based suspension of silica nanoparticles on flat glass substrates. The main purpose of this Optics Materials Express paper is to use laser processing techniques to improve the formation of these optical microwires. A brief description of the authors’ method is as follows. First, the authors employed a UV laser with a wavelength of 193 nm to alter the surface properties of borosilicate slide. By changing the laser fluence and the separation between scanning lines, the authors could control the morphological profile of the slides, resulting in a change in the contact angle exceeding 25°. Next, by selectively producing surface structures on the slides, the authors were able to convert the shape of the drop on the slide surface from a spherical caplet to an approximate ellipsoid caplet by inducing asymmetric contact angle distribution, and tailor the convective flow within the drop so that the radial stresses within the caplet are focused at two points. This tailoring of convective flow eventually led to the formation of the optical microwires more uniformly.

Overall, the authors suggested a new method of controlling the fabrication of the optical microwires with the help of a laser processing technique. However, the main discovery of this paper is not limited to the improvement of fabrication of the optical microwires. This research may be able to open up a new direction of microfluidics combined with laser processing, since sophisticated control of liquid flow on a surface can be achieved through surface structuring with laser irradiation.